1. Field of the Invention
The present invention generally relates to an image forming method used in ultrasonic diagnoses, and also to an ultrasonic diagnostic apparatus with employment of this image forming method. More specifically, the present invention is directed to such an image forming method available in ultrasonic diagnoses, while using ultrasonic waves having a plurality of continuous cycles (wavelengths). Furthermore, the present invention is related to a signal processing apparatus capable of processing a detection signal which is acquired by detecting ultrasonic waves, while forming an ultrasonic diagnostic image used in such an ultrasonic diagnosis, and also related to a recording medium for recording thereon a signal processing program.
2. Description of a Related Art
Currently, since ultrasonic diagnoses are capable of acquiring blood flow information as featured functions, these ultrasonic diagnoses could be considerably advanced with respect to chest portions and abdomen portions of living bodies, or patients. In particular, since ultrasonic imaging techniques using contrast media have been recently developed, more precise blood flow information may be acquired. In such an ultrasonic contrast imaging operation, a micro bubble contrast medium is mainly injected into a vein. This micro bubble contrast medium is produced by mixing a large number of micro bubbles whose diameters are selected to be 1 micrometer to several micrometers into a fluid. This micro bubble is made in such a manner that a harmless gas for a living body (for example, air, carbon fluoride, etc.) is filled into a shell made of a harmless substance for a living body (for instance, lecithin).
Japanese Laid-open Patent Application JP-A-9-164138 describes the ultrasonic diagnostic image processing method. That is, while the ultrasonic contrast medium made of very fine bubbles is injected into the blood flows and also ultrasonic pulses capable of breaking the very fine bubbles contained in the tissue are transmitted, this ultrasonic diagnostic image processing method can check as to how degree, the very fine bubbles are re-circulated within the tissue during a time duration by measuring the ultrasonic waves. This time duration is defined after the very fine bubbles are broken and a certain time interval has elapsed.
Also, in ultrasonic diagnostic image forming techniques, Doppler signals or higher harmonic signals are advantageously utilized, and thus, blood flow information may be acquired from larger numbers of tissue of living bodies. In particular, since such an ultrasonic diagnostic image forming technique is combined with an ultrasonic contrast imaging technique, movement of blood flows can be more precisely estimated.
On the other hand, Japanese Laid-open Patent Application JP-A-11-178824 discloses the pulse inversion Doppler ultrasonic diagnostic image processing method. This pulse inversion Doppler ultrasonic diagnostic image processing method is realized by employing a transmission step of transmitting a sequence of modulated ultrasonic waves into the human body, and of producing the phase difference to the ultrasonic echoes acquired in response to this ultrasonic wave transmission; a reception step of receiving a set of the ultrasonic echo signals in response to the transmitted sequence; and a analysis step of analyzing this signal set in order to separate the phase shift information of the linear signal component from the phase shift information of the non-linear signal component.
However, while such a Doppler signal is detected, since the strong signals originated from the tissue having large movement (e.g., cardiac muscle) and also the higher harmonic signals generated from the tissue itself are mixed into this Doppler signal, it is impossible to detect the micro bubbles present in the blood vessels only, by this pulse inversion Doppler ultrasonic diagnostic image processing method.
Also, U.S. Pat. No. 5,706,819 describes such an ultrasonic diagnostic image processing method that since the influence caused by the higher harmonic contrast medium is received while alternately inverting the polarity (phase), the higher harmonic component of the transmission signal can be suppressed and also the scattering components can be removed so as to detect the influence of the higher harmonic contrast medium.
However, in order to execute such a higher harmonic image processing operation, a plurality of ultrasonic waves having the different polarities (different phases) are required to be transmitted. As a result, lengthy measuring time would be required. When the object under measurement is moved during such lengthy measuring time, there is such a problem that the space resolution of the image would be lowered.
On the other hand, a so-called xe2x80x9csub-harmonic imagingxe2x80x9d technique may be gradually brought into consideration. In this sub-harmonic imaging technique, while such an ultrasonic wave containing a plurality of continuous waves is transmitted to an object to be inspected, an image may be formed based upon sub-harmonic echoes which are produced only from micro bubbles contained in a blood vessel. Since a sub-harmonic component is produced only by chaos-like vibrations as well as a branching phenomenon of micro bubbles, the following merit may be conceived in accordance with such a sub-harmonic imaging technique. That is to say, images having higher contrast than that of a harmonic imaging technique may be produced in this sub-harmonic imaging technique.
With respect to sub-harmonic echoes of micro bubbles, such sub-harmonic echoes can be produced by using continuous ultrasonic waves, which is known from the document xe2x80x9cSubharmonic backscattering from ultrasound contrast agentsxe2x80x9d written by P. M. Shankar et. al., J. Acoust. Soc. Am. 106(4), 1999, p. 2104-2110. To execute such a sub-harmonic imaging technique, an ultrasonic wave containing a plurality of continuous waves called as a burst wave may be employed.
However, when such a plurality of waves which are continued for a long time duration such as a burst wave are employed, one set of waves becomes long. As a result, there is another problem that the resulting space resolution of the image is lowered.
Also, Japanese Laid-open Patent Application JP-A-2000-5167 discloses the ultrasonic wave transmitting method. That is, while the ultrasonic wave having a plurality of continuous waves is transmitted, such an ultrasonic wave is transmitted in order to firmly produce sub-harmonic echoes. In this ultrasonic wave, there are provided such ultrasonic waves having the instantaneous sound pressure capable of breaking the micro balloons (micro bubbles) before/after at least one ultrasonic wave having the instantaneous sound pressure incapable of breaking the micro balloons.
However, the transmission/reception of the ultrasonic wave with employment of the sub-harmonic echoes still requires further improvements including the process operation of the detection signals.
As a method of detecting a sub-harmonic strength, such a method of FFT (fast Fourier transform)-processing a received waveform has been employed. However, since this FFT method requires length calculation time, there is such a problem that this FFT method is not suitable for such a real-time image display operation. On the other hand, in accordance with the method of deriving a frequency component of a sub-harmonic wave by filtering a detection signal by a filtering circuit, frequency components of such a sub-harmonic wave are sandwiched among fundamental waves and higher harmonic waves, and thus, these frequency components are present over a plurality of frequencies. As a consequence, it is practically difficult to extract only such sub-harmonic wave components.
The present invention has been made to solve these problems, and therefore, has a first object to provide both an image forming method and an ultrasonic diagnostic apparatus, capable of acquiring an image having superior space resolution in ultrasonic diagnoses.
Also, a second object of the present invention is to provide both an image forming method and an ultrasonic diagnostic apparatus, capable of producing sub-harmonic echoes in high certainty, and also capable of displaying sub-harmonic information in such a higher speed nearly equal to real time in ultrasonic diagnoses. Furthermore, a third object of the present invention is to provide a signal processing apparatus and a recording medium for recording thereon a signal processing program, which may be utilized in such an image forming operation.
To achieve the above-explained objects, an image forming method to be used in an ultrasonic diagnosis according to the present invention includes the steps of: (a) transmitting an ultrasonic wave, which is continued for at least two cycles, to an object to be inspected; (b) detecting an echo signal which is produced by reflection of the transmitted ultrasonic wave from tissue of the object to be inspected to thereby obtain a detection signal; (c) delaying the detection signal by such a time duration corresponding to one cycle of the transmitted ultrasonic wave to thereby obtain a delayed detection signal; and (d) obtaining image information related to the tissue of the object to be inspected based upon a difference between the detection signal and the delayed detection signal.
Also, a ultrasonic diagnostic apparatus according to the present invention includes: first means for transmitting an ultrasonic wave, which is continued for at least two cycles, to an object to be inspected, and for detecting an echo signal which is produced by reflection of the transmitted ultrasonic wave from tissue of the object to be inspected to thereby obtain a detection signal; second means for delaying the detection signal by such a time duration corresponding to one cycle of the transmitted ultrasonic wave to thereby obtain a delayed detection signal; and third means for obtaining image information related to the tissue of the object to be inspected based upon a difference between the detection signal and the delayed detection signal.
Furthermore, a signal processing apparatus according to the present invention for processing a detection signal which is obtained by that an ultrasonic wave continued for at least two cycles is transmitted to an object to be inspected and an echo signal produced by reflection of the transmitted ultrasonic wave from tissue of the object to be inspected is detected, includes: first means for delaying the detection signal by such a time duration corresponding to one cycle of the transmitted ultrasonic wave to thereby obtain a delayed detection signal; and second means for obtaining image information related to the tissue of the object to be inspected based upon a difference between the detection signal and the delayed detection signal.
In addition, a recording medium according to the present invention for recording signal processing program is readable by a CPU (central processing unit) and is recording a signal processing program for processing a detection signal which is obtained by that an ultrasonic wave continued for at least N/2 cycles within a predetermined time duration is transmitted to an object to be inspected and an echo signal produced by reflection of the transmitted ultrasonic wave from tissue of the object to be inspected is detected where N is an integer not less than four, the signal processing program causing the CPU to execute: delaying the detection signal by such a time duration corresponding to one cycle of the transmitted ultrasonic wave to thereby obtain a delayed detection signal; obtaining a difference signal between the detection signal and the delayed detection signal; obtaining, when N is equal to an even number, image information related to the tissue of the object to be inspected based upon a difference between the difference signal within a first predetermined time duration and the difference signal within a second predetermined time duration; and obtaining, when N is equal to an odd number, image information related to the tissue of the object to be inspected based upon a summation of the difference signal within the first predetermined time duration and the difference signal within the second predetermined time duration.